Description The horseshoe crab (Limulus polyphemus) is a ‘living fossil’: forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, a staggering 400 million years ago. Despite their common name, horseshoe crabs are not crabs but are related to arachnids (spiders, scorpions, ticks and mites), and are the closest living relatives of the now extinct trilobites. Horseshoe crabs have three main parts to the body: the head region, known as the ‘prosoma’, the abdominal region or